It's not a limitation of VCSM (which will allocate any multiple of the standard 4kB page size), but potentially of the EGL import.
The person who wrote that code isn't particularly active these days, but if you can produce a simple test case then I can try running it and tracing through what happens in the firmware. I don't know how strict that power of 2 limitation is, or why it is required.

The TMUs require mipmaps to be stored before the level 0 texture in memory, with addresses automatically
determined assuming power of 2 sized images.The TMU unit supports mipmapped non-power-of-two textures,
but the mipmap levels greater than 0 must be padded to fit within a power of 2 container image.

Note that the buffer must be PADDED to a power of 2, not that the number of active pixels is a power of 2. Yes it's extra memory, but shouldn't require extra processing.